1. Field of the Invention
The present invention relates to multilayer systems.
2. Description of the Prior Art
Multilayer sytems take the form of a substrate which supports a plurality of layers of electrical conductors separated from one another by layers of insulation but selectively interconnected to one another by means of through connections in the layers of insulation.
In a previously proposed system each layer of conductors is formed on a substrate and covered with a dielectric layer which then forms the substrate for the next conductive layer. Through holes are formed in the dielectric layer to enable interconnections between adjacent conductive layers. Such through holes may be formed at the time of screen printing if the conductors are also screen printed or instead be formed later for example by laser drilling.
The conductive layer is formed by screen printing a thick film (as the technique is commonly known) of particulate conductive material on the substrate. The conductive film is then heated to promote sintering and adhesion of the layer to the substrate. Thereafter the whole is coated with a glass forming dielectric and then fired in a furnace at a temperature from the range of 600.degree. C. and to 900.degree. C. to melt the dielectric and to promote adhesion of the dielectric to the substrate. The conductive material may be of copper or copper based materials. The screen printing process is, however, generally limited in resolution. Copper and copper based materials, because they are deposited as small particles and not as a solid layer, are subject to oxidation throughout the layer and therefore require processing in an inert environment for example of nitrogen to inhibit oxidation. In this case dielectric glass forming materials must also be processed in a nitrogen environment and these are generally difficult to fabricate.
Furnaces providing a nitrogen environment are complex and expensive to run. Not all dielectrics are suitable for firing in a nitrogen environment, and those that are tend to become somewhat porous. Even if the insulator is built up layer by layer the desired density is not achieved and the insulator remains porous.
It is an object of the invention to provide an improved multilayer system.